1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method for irradiating a light-emitting substance with electrons.
2. Description of the Related Art
A technique of forming an image by irradiating a light-emitting substance with electrons has conventionally been known. A well-known example of this technique is a CRT.
Two types of devices, namely hot and cold cathode devices, are known as electron-emitting devices. Known examples of the cold cathode devices are surface-conduction type electron-emitting devices, field emission type electron-emitting devices (to be referred to as FE type electron-emitting devices hereinafter), and metal/insulator/metal type electron-emitting devices (to be referred to as MIM type electron-emitting devices hereinafter).
A known example of the surface-conduction type electron-emitting devices is described in, e.g., M. I. Elinson, xe2x80x9cRadio Eng. Electron Phys., 10, 1290 (1965) and other examples will be described later.
The surface-conduction type electron-emitting device utilizes the phenomenon that electrons are emitted by a small-area thin film formed on a substrate by flowing a current parallel through the film surface. The surface-conduction type electron-emitting device includes electron-emitting devices using an Au thin film [G. Dittmer, xe2x80x9cThin Solid Filmsxe2x80x9d, 9,317 (1972)], an In2O3/SnO2 thin film [M. Hartwell and C. G. Fonstad, xe2x80x9cIEEE Trans. ED Conf.xe2x80x9d, 519 (1975)], a carbon thin film [Hisashi Araki et al., xe2x80x9cVacuumxe2x80x9d, Vol. 26, No. 1, p. 22 (1983)], and the like, in addition to an SnO2 thin film according to Elinson mentioned above.
FIG. 17 is a plan view showing the device by M. Hartwell et al. described above as a typical example of the device structures of these surface-conduction type electron-emitting devices. Referring to FIG. 17, reference numeral 3001 denotes a substrate; and 3004, a conductive thin film made of a metal oxide formed by sputtering. This conductive thin film 3004 has an H-shaped pattern, as shown in FIG. 17. An electron-emitting portion 3005 is formed by performing electrification processing (referred to as forming processing to be described later) with respect to the conductive thin film 3004. An interval L in FIG. 17 is set to 0.5 to 1 mm, and a width W is set to 0.1 mm. The electron-emitting portion 3005 is shown in a rectangular shape at the center of the conductive thin film 3004 for the sake of illustrative convenience. However, this does not exactly show the actual position and shape of the electron-emitting portion.
In the above surface-conduction type electron-emitting devices by M. Hartwell et al. and the like, typically the electron-emitting portion 3005 is formed by performing electrification processing called forming processing for the conductive thin film 3004 before electron emission. In the forming processing, an electron-emitting portion is formed by electrification such that a constant DC voltage or a DC voltage which increases at a very low rate of, e.g., 1 V/min is applied across the two ends of the conductive thin film 3004 to partially destroy or deform the conductive thin film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 3004 has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after the forming processing, electrons are emitted near the fissure.
Known examples of the FE type electron-emitting devices are described in W. P. Dyke and W. W. Dolan, xe2x80x9cField emissionxe2x80x9d, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, xe2x80x9cPhysical properties of thin film field emission cathodes with molybdenium conesxe2x80x9d, J. Appl. Phys., 47, 5248 (1976).
FIG. 18 is a sectional view showing the device by C. A. Spindt et al. described above as a typical example of the FE type device structure. In FIG. 18, reference numeral 3010 denotes a substrate; 3011, an emitter wiring made of a conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. In this device, a voltage is applied between the emitter cone 3012 and gate electrode 3014 to emit electrons from the distal end portion of the emitter cone 3012.
As another FE type device structure, there is an example in which an emitter and gate electrode are arranged on a substrate to be almost parallel to the surface of the substrate, in addition to the multilayered structure of FIG. 18.
A known example of the MIM type electron-emitting devices is described in C. A. Mead, xe2x80x9cOperation of Tunnel-Emission Devicesxe2x80x9d, J. Appl. Phys., 32,646 (1961). FIG. 19 shows a typical example of the MIM type device structure. FIG. 19 is a sectional view of the MIM type electron-emitting device. In FIG. 19, reference numeral 3020 denotes a substrate; 3021, a lower electrode made of a metal; 3022, a thin insulating layer having a thickness of about 100 xc3x85; and 3023, an upper electrode made of a metal and having a thickness of about 80 to 300 xc3x85. In the MIM type electron-emitting device, an appropriate voltage is applied between the upper and lower electrodes 3023 and 3021 to emit electrons from the surface of the upper electrode 3023.
Of cold cathode devices, the above surface-conduction type electron-emitting devices have a simple structure and can be easily manufactured, and thus many devices can be formed on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving a lot of devices has been studied.
Regarding applications of the surface-conduction type electron-emitting devices to, e.g., image forming apparatuses such as an image display apparatus and an image recording apparatus, charge beam sources, and the like have been studied.
Particularly as an application to image display apparatuses, as disclosed in the U.S. Pat. No. 5,066,833 and Japanese Patent Laid-Open Nos. 2-257551 and 4-28137 filed by the present applicant, an image display apparatus using the combination of an surface-conduction type electron-emitting device and a fluorescent substance which emits light upon reception of an electron beam has been studied. This type of image display apparatus using the combination of the surface-conduction type electron-emitting device and the fluorescent substance is expected to exhibit more excellent characteristics than other conventional image display apparatuses. For example, compared with recent popular liquid crystal display apparatuses, the above display apparatus is superior in that it does not require any backlight because it is of a self-emission type and that it has a wide view angle.
A method of driving a plurality of FE type electron-emitting devices arranged side by side is disclosed in, e.g., U.S. Pat. No. 4,904,895 filed by the present applicant. As a known example of an application of FE type electron-emitting devices to an image display apparatus is a flat display apparatus reported by R. Meyeret al. [R. Meyer: xe2x80x9cRecent Development on Microtips Display at LETIxe2x80x9d, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)].
An example of an application of a larger number of MIM type electron-emitting devices arranged side by side to an image display apparatus is disclosed in Japanese Patent Laid-Open No. 3-55738 filed by the present applicant.
The present inventors have examined cold cathode devices of various materials, various manufacturing methods, and various structures, in addition to the above-mentioned conventional cold cathode devices. Further, the present inventors have made extensive studies on a multi electron beam source having a large number of cold cathode devices, and an image display apparatus using this multi electron beam source.
The present inventors have examined a multi electron beam source having an electrical wiring method shown in, e.g., FIG. 20. That is, a large number of cold cathode devices are two-dimensionally arranged in a matrix to obtain a multi electron beam source, as shown in FIG. 20.
Referring to FIG. 20, reference numeral 4001 denotes a cold cathode device; 4002, a row-direction wiring; and 4003, a column-direction wiring. The row- and column-direction wirings 4002 and 4003 actually have finite electrical resistances, which are represented as wiring resistances 4004 and 4005 in FIG. 20. This wiring method is called a simple matrix wiring method.
For the illustrative convenience, the multi electron beam source is illustrated in a 6xc3x976 matrix, but the size of the matrix is not limited to this. For example, in amulti beam electron source for an image display apparatus, the number of devices enough to perform desired image display are arranged and wired.
In a multi electron beam source constituted by arranging cold cathode devices in a simple matrix, appropriate electrical signals are applied to the row- and column-direction wirings 4002 and 4003 to output a desired electron beam. For example, to drive the cold cathode devices on an arbitrary row in the matrix, a selection voltage Vs is applied to the column-direction wiring 4002 on the row to be selected, and at the same time a non-selection voltage Vns is applied to the row-direction wirings 4002 on an unselected row. In synchronism with this, a driving voltage Ve for outputting an electron beam is applied to the column-direction wiring 4003. According to this method, when voltage drops across the wiring resistances 4004 and 4005 are neglected, a voltage (Vexe2x88x92Vs) is applied to the cold cathode devices on the selected row, while a voltage (Vexe2x88x92Vns) is applied to the cold cathode devices on the unselected row. When the voltages Ve, Vs, and Vns are set to appropriate magnitudes, an electron beam having a desired intensity must be output from only the cold cathode device on the selected row. When different driving voltages Ve are applied to respective column-direction wirings, electron beams having different intensities must be output from the respective devices of the selected row. A change in length of time for which the driving voltage Ve is applied necessarily causes a change in length of time for which an electron beam is output.
The multi electron beam source constituted by arranging cold cathode devices in a simple matrix has a variety of applications. For example, when an electrical signal corresponding to image information is appropriately applied, the multi electron beam source can be suitably used as an electron source for an image display apparatus.
The present invention provides a new apparatus and method for forming an image using an electron-emitting device.
More specifically, the conventional apparatus and method suffer the following problems.
Problem (1): The number of devices must be increased to display a high-quality image. Along with this, demands arise for integrating driving circuits for driving many devices and reducing power consumption of the drivers.
Problem (2): The emission/non-emission threshold voltage may slightly vary depending on the accelerating voltage of an emitted electron beam, the panel lot, or the like.
Problem (3): In some cases, the user wants to change the peak luminance in accordance with the ambient brightness at the installation location of the image display apparatus or user tastes.
Further, the user wants to lower the peak luminance in order to suppress power consumption of the image display apparatus.
Problem (4): In some cases, the user wants to set the peak luminance in accordance with a corresponding input image signal when the image display apparatus displays a plurality of image signals such as a TV signal and computer output image signal.
Problem (5): A TV signal is generally received by a receiver using a CRT. The TV signal is output after the gamma characteristic (nonlinear characteristic of luminance signal vs. emission luminance characteristic) of the CRT is corrected (to be referred to as gamma correction hereinafter) on the transmission side in advance.
In other words, when a TV signal is received by a display apparatus using a display device other than the CRT, like the image display apparatus, the display apparatus must adopt an emission characteristic conversion means for adjusting the emission characteristic of the display device to the nonlinear one of the CRT.
Problem (6): To display a color image on the image apparatus, a display panel having three types of fluorescent substances which emit light in red, green, and blue is formed. However, the emission amount of the fluorescent substance which emits light by irradiation of an electron beam varies depending on the type of fluorescent substance in use or the accelerating voltage of an electron beam. Preferable color reproducibility is not always obtained by irradiating the three, red, green, and blue types of fluorescent substances with the same amount of electron beam. In some cases, the user wants to control the irradiation beam amount in accordance with the accelerating voltage or a fluorescent substance in use.
Moreover, the user wants to change the emission color tone of the image display apparatus in accordance with the color tone of the ambient light at the installation location of the image display apparatus or user tastes, or to change the emission color tone in accordance with the type of signal input to the image display apparatus.
In this case, the emission color tone means:
an emission color when the luminance level of an input signal is as low as almost black;
an emission color when the luminance level of an input signal is as high as almost the maximum emission luminance; and
an emission color when an input luminance signal changes from black to white.
Problem (7): When the image display apparatus is constituted using an electron-emitting device, particularly a cold cathode device, and more particularly a surface-conduction type electron-emitting device, like the present invention, an emitted electron beam, i.e., luminance can be modulated by controlling the device application voltage, as described above. However, the electron-emitting device has a rated voltage at which device characteristics degrade or cannot be guaranteed upon application of a device application voltage equal to or higher than a certain voltage value. Hence, this image display requires a protection means for avoiding application of a device voltage equal to or higher than the rated voltage.
One aspect of the image forming apparatus according to the present invention has the following arrangement.
There is provided an image forming apparatus comprising
a plurality of electron-emitting devices arranged in a matrix using pluralities of first and second wirings,
a light-emitting substance for emitting light by irradiation of electrons emitted by the electron-emitting devices,
a first wiring driving circuit for sequentially selecting the plurality of first wirings and applying, to a selected first wiring, a predetermined potential different from a potential to an unselected first wiring, and
a second wiring driving circuit for applying a potential corresponding to an image signal to the plurality of second wirings,
characterized in that a potential difference between potentials applied by the first and second wirings to an electron-emitting device which is connected to the first wiring selected by the first wiring driving circuit and is not required to emit light from the light-emitting substance by irradiation of electrons from the electron-emitting device is around a threshold of emission/non-emission of the light-emitting substance by irradiation of electrons from the electron-emitting device.
This arrangement can reduce power consumption. The present inventor has found that power consumption in a circuit for outputting a potential which changes in accordance with an image signal is larger than power consumption in a circuit for sequentially selecting a plurality of wirings (first wirings). This is because the circuit for outputting a potential which changes in accordance with an image signal must output a potential which changes in accordance with, e.g., a desired luminance, whereas the circuit for sequentially selecting a plurality of wirings (first wirings) only performs simple control. (For example, the latter circuit can be formed from a switching circuit such as a transistor when the circuit is switched between selection and non-selection by a binary potential.) Based on this finding, the present inventor has found to reduce power consumption by setting the potential difference between potentials applied by the first and second wirings to an electron-emitting device which is connected to the first wiring selected by the first wiring driver and is not required to emit light from the light-emitting substance by irradiation of electrons from the electron-emitting device, close to the threshold of emission/non-emission of the light-emitting substance by irradiation of electrons from the electron-emitting device. This is because this arrangement can narrow the change range of an output potential from the circuit for outputting a potential which changes in accordance with an image signal.
The threshold can be determined using any one of the followings.
1) The threshold is set to a potential difference when a potential difference is applied to an electron-emitting device by the first and second wirings, and the luminance at the emission position of a light-emitting substance corresponding to the electron-emitting device has a significant value. The luminance can have a significant value when light emitted by the light-emitting substance can be visually recognized with a naked eye in a very dark environment such as a darkroom.
2) The threshold is set to a potential difference when the luminance exceeds the luminance of ambient light. According to xe2x80x9cEvaluation Technique for Television Imagexe2x80x9d, Corona-Sha, pp. 83-85, the luminance of the display surface of a receiver by ambient light in home is estimated to be about 2 to 3 cd/m2. The luminance can be measured by a luminance meter.
3) The threshold is set to a different potential having a luminance of 2 cd/m2. The luminance can be measured by a luminance meter.
4) The threshold is determined by a contrast ratio.
Assuming that the peak luminance of the display apparatus is L cd/m2, and the contrast ratio is k, the threshold is set to a potential difference when the luminance reaches L/k cd/m2. According to the reference, a desirable contrast ratio is 30 or more under use conditions in home. Under darkroom conditions free from any influence of external light, adesirable contrast ratio is 100 or more in many cases. For example, if the peak luminance is 300 cd/m2 and the contrast ratio necessary for a darkroom is 200, a potential difference having a luminance of 1.5 cd/m2 is set as a threshold. In the viewpoint of contrast, the potential difference between potentials applied by the first and second wirings to an electron-emitting device not required to emit light is desirably the threshold or less.
The potential difference applied to an electron-emitting device which is connected to the selected first wiring and is not required to emit light from the light-emitting substance by irradiation of electrons from the electron-emitting device is set around the threshold so as to make the difference between the potential difference and threshold fall within 10% of the threshold, preferably 5%, and more preferably 1%. An embodiment of the present invention may set the potential difference larger than the threshold. Also in this case, the difference between the potential difference and threshold is made to fall within 10% of the threshold, preferably 5%, and more preferably 1%.
A potential applied to the electron-emitting device can be obtained by a potential applied to the wiring, the resistance of the wiring, and a current value flowing through the wiring.
The above aspect can be constituted such that the predetermined potential applied to the selected first wiring is lower than a predetermined reference potential by a predetermined value and the potential applied to each second wiring is not less than the reference potential, or the predetermined potential applied to the selected first wiring is higher than a predetermined reference potential by a predetermined value and the potential applied to each second wiring is not more than the reference potential, the first wiring driving circuit applies the predetermined potential to the selected first wiring and the reference potential to the unselected first wiring, and a potential difference between the reference potential and the predetermined potential applied to the selected first wiring is around the threshold. A preferable reference potential is a ground potential.
The arrangement of setting one of potentials within the change range applied to the second wiring that is closest to the potential applied to the selected first wiring to be almost equal to the potential applied to the unselected first wiring is exemplified in, e.g., the first embodiment. In the first embodiment, the reference potential is 0 V, and the potential applied to the selected first wiring (i.e., row wiring) is xe2x88x9211 V. A potential corresponding to an image signal applied to each second wiring (column wiring) falls within the range of 0 V to 4 V.
The first aspect can be constituted such that the image forming apparatus further comprises means for applying a predetermined potential to each second wiring, and a potential difference between the predetermined potential applied to each second wiring and the predetermined potential applied to the selected first wiring is around the threshold. An unstable potential state of the second wiring can be avoided by applying a predetermined potential to a second wiring connected to an electron-emitting device which is connected to the first wiring selected by the first wiring driving circuit and is not required to emit light by irradiation of electrons from the electron-emitting device, and/or a second wiring when no potential corresponding to an image signal is applied.
This aspect can also preferably employ an arrangement in which a potential applied to a second wiring connected to an electron-emitting device which is connected to a first wiring selected by the first wiring driving circuit and is not required to emit light by irradiation of electrons from the electron-emitting device is different from a potential applied to an unselected first wiring. In this case, the potential difference between the potential applied to the second wiring connected to the electron-emitting device which is connected to the selected first wiring and is not required to emit light by irradiation of electrons from the electron-emitting device, and the potential applied to the unselected first wiring does not substantially contribute to the luminance of the light-emitting substance. This potential difference is called an offset voltage. A circuit for applying a potential for this offset voltage is preferably arranged separately from a circuit for controlling a potential applied to the second wiring in accordance with a luminance level.
This arrangement is exemplified in, e.g., the second embodiment. The potential applied to the selected first wiring is xe2x88x9210.5 V, and the predetermined potential applied to each second wiring is 0.5 V. In the second embodiment as well as the first embodiment, a potential corresponding to an image signal is controlled within the range of 0 V to 4 V. Thus, the second wiring receives a potential change from 0.5 V to 4.5 V in addition to the predetermined potential of 0.5 V.
Each aspect may comprise means for adjusting the potential difference between the potentials applied by the first and second wirings to the electron-emitting device which is connected to the first wiring selected by the first wiring driving circuit and is not required to emit light from the light-emitting substance by irradiation of electrons from the electron-emitting device. The potential difference can be adjusted by adjusting the predetermined potential applied to the selected first wiring. In the arrangement of applying the offset voltage, the potential difference can be adjusted by adjusting an offset potential for applying the offset voltage.
Each aspect may comprise means for adjusting a change range of a potential corresponding to an image signal. Since the change range of the potential corresponding to the image signal can be adjusted, the peak luminance can be adjusted. Consequently, the peak luminance can be adjusted in accordance with the ambient brightness of the image forming apparatus or user tastes, or in order to suppress power consumption.
Each aspect may comprise means for determining a type of input image signal and adjusting a change range of a potential corresponding to the image signal on the basis of a determination result. This arrangement allows adjusting the peak luminance in accordance with the type of input image signal such as a TV signal or computer output image signal. Each aspect may comprise means for determining the type of input image signal.
Each aspect may comprise a circuit for detecting a current value flowing through the electron-emitting device and means for adjusting a change range of a potential corresponding an image signal on the basis of a detection result. As the arrangement of detecting a current value flowing through the electron-emitting device, an arrangement of detecting a current emitted by the electron-emitting device is preferably adopted. When, for example, the emitted current value becomes excessively large, the current value can be suppressed.
Each aspect may comprise means for detecting an average luminance level of an input image signal and adjusting a change range of a potential corresponding to the image signal on the basis of a detection result. This arrangement allows adjusting the peak luminance in accordance with the average luminance level. Especially when the average luminance level is high, the peak luminance can be adjusted to a low value, thereby suppressing power consumption. Each aspect may comprise means for detecting an average luminance level of an input image signal.
In each aspect, if any one or a plurality of (1) a potential applied to the selected first wiring, (2) the change range of the potential corresponding to the image signal, and (3) the predetermined potential in the arrangement capable of applying the predetermined potential (including a potential for applying the offset voltage) to each second wiring are adjusted, the total voltage applied to the electron-emitting device may exceed the allowable range to degrade the characteristics of the electron-emitting device or fail to guarantee electron-emitting characteristics. To avoid this, each aspect desirably comprises means for defining the upper limit of a voltage applied to the electron-emitting device. Alternatively, each aspect may comprise means for adjusting the remaining one of (1) to (3) in accordance with a change in any one of (1) to (3).
Each aspect may comprise means for correcting the potential difference between the potentials applied by the first and second wirings to the electron-emitting device on the basis of a relationship between the potential difference and an emission luminance of the light-emitting substance by irradiation of electrons from the electron-emitting device which receives the potential difference. This correction means allows forming a more accurate image. In particular, a normal TV signal is output after the gamma characteristic (nonlinear relationship between a luminance signal and emission luminance) is corrected (to be referred to as gamma correction hereinafter). Therefore, the signal is corrected in accordance with the characteristics of an electron-emitting device or light-emitting substance in use, thereby forming a more accurate image.
To attain multi-color display in each aspect, light-emitting substances are formed for respective colors. Specifically, three, red, green, and blue light-emitting substances are preferably formed to adjust the luminance for the respective colors. In multi-color display, the emission amount of the light-emitting substance which emits light by irradiation of an electron beam changes depending on the type of light-emitting substance in use and the accelerating voltage of an electron beam. A desired display is not always obtained by irradiating the different types of light-emitting substances with the same amount of electron beam. In this case, a potential applied by the second wiring to the electron-emitting device is adjusted in accordance with a color corresponding to each electron-emitting device. More specifically, the change range of a potential applied by the second wiring is adjusted for each color. In the arrangement capable of applying a predetermined potential (including a potential for applying the offset voltage) to each second wiring, the predetermined potential may be adjusted for each color. Alternatively, the above-described correction may be done for each color. While one first wiring is selected, an effective length of a time for applying a potential associated with emission by the second wiring to an electron-emitting device connected to the selected first wiring may be adjusted in accordance with a color corresponding to each electron-emitting device.
In each aspect, a type of input image signal may be determined, and while one first wiring is selected, an effective length of a time for applying a potential associated with emission by the second wiring to an electron-emitting device connected to the selected first wiring may be adjusted on the basis of a determination result.
In each aspect, an average luminance level of an input image signal may be detected, and while one first wiring is selected, an effective length of a time for applying a potential associated with emission by the second wiring to an electron-emitting device connected to the selected first wiring may be adjusted on the basis of a detection result.
In each aspect, a current value flowing through the electron-emitting device may be detected, and while one first wiring is selected, an effective length of a time for applying a potential associated with emission by the second wiring to an electron-emitting device connected to the selected first wiring may be adjusted on the basis of a detection result.
In these arrangements, an effective length of a time for applying a potential associated with emission by the second wiring may be adjusted for each color of the light-emitting substance in accordance with the type of image signal and the average luminance level.
In each aspect, the electron-emitting device may be a cold cathode device. Since the cold cathode device can emit electrons at a lower temperature than a hot cathode device, it does not require any heater. The cold cathode is simpler in structure than the hot cathode device and can shrink in feature size. Even a large number of devices can be arranged at a high density. A high-density arrangement suffers particularly thermal problems, but the cold cathode device is almost free from these problems. In addition, the response speed of the hot cathode device is low because it operates upon heating. To the contrary, the response speed of the cold cathode device is high.
In each aspect, the electron-emitting device may be a surface-conduction type electron-emitting device. The surface-conduction type electron-emitting device is simple in structure and can be easily manufactured.
In each aspect, a potential corresponding to an image signal that is applied to the second wiring may be controlled in accordance with a luminance level.
In each aspect, a change range of a potential applied for luminance gray scale display by the second wiring to an electron-emitting device connected to a first wiring selected by the first wiring driving circuit is preferably narrower than a potential difference between one of potentials within the potential change range that is closest to a potential applied to the selected first wiring, and the potential applied to the selected first wiring.
Another aspect of the image forming apparatus according to the present invention has the following arrangement.
There is provided an image forming apparatus comprising
a plurality of electron-emitting devices,
a light-emitting substance for emitting light by irradiation of electrons emitted from the electron-emitting devices,
first potential application means for sequentially selecting the plurality of electron-emitting devices and applying, to a selected electron-emitting device, a predetermined potential different from a potential applied to an unselected electron-emitting device, and
second potential application means for applying a potential corresponding to an image signal to at least a selected electron-emitting device,
characterized in that a potential difference between potentials applied by the first and second potential application means to the selected electron-emitting device is around a threshold of emission/non-emission of the light-emitting substance by irradiation of electrons from the electron-emitting device when the light-emitting substance is not required to emit light by irradiation of electrons from the selected electron-emitting device.
Still another aspect of the image forming apparatus according to the present invention has the following arrangement.
There is provided an image forming apparatus comprising
a plurality of electron-emitting devices,
an light-emitting substance for emitting light by irradiation of electrons emitted from the electron-emitting devices,
first potential application means for sequentially selecting the plurality of electron-emitting devices and applying, to a selected electron-emitting device, a predetermined potential different from a potential applied to an unselected electron-emitting device, and
second potential application means for applying a potential corresponding to an image signal to at least a selected electron-emitting device for luminance gray scale display,
characterized in that the lowest luminance in luminance gray scale display is an light emission level.
The electron-emitting device in each aspect suffices to receive at least two potentials and emit electrons by the potential difference between these potentials. The number of electron-emitting devices is set to a necessary one for forming a desired image. One electron-emitting device preferably corresponds to one pixel.
One aspect of the image forming method according to the present invention has the following steps.
There is provided an image forming method in an image forming apparatus having
a plurality of electron-emitting devices arranged in a matrix using pluralities of first and second wirings,
a light-emitting substance for emitting light by irradiation of electrons emitted by the electron-emitting devices,
a first wiring driving circuit for sequentially selecting the plurality of first wirings and applying, to a selected first wiring, a predetermined potential different from a potential to an unselected first wiring, and
a second wiring driving circuit for applying a potential corresponding to an image signal to the plurality of second wirings,
characterized in that a potential difference between potentials applied by the first and second wirings to an electron-emitting device which is connected to the first wiring selected by the first wiring driving circuit and is not required to emit light from the light-emitting substance by irradiation of electrons from the electron-emitting device is around a threshold of emission/non-emission of the light-emitting substance by irradiation of electrons from the electron-emitting device, and an image is formed by applying a potential corresponding to an image signal to the plurality of second wirings while sequentially selecting the plurality of first wirings.
Another aspect of the image forming method according to the present invention has the following steps.
There is provided an image forming method in an image forming apparatus having
a plurality of electron-emitting devices,
a light-emitting substance for emitting light by irradiation of electrons emitted from the electron-emitting devices,
first potential application means for sequentially selecting the plurality of electron-emitting devices and applying, to a selected electron-emitting device, a predetermined potential different from a potential applied to an unselected electron-emitting device, and
second potential application means for applying a potential corresponding to an image signal to at least a selected electron-emitting device,
characterized in that a potential difference between potentials applied by the first and second potential application means to the selected electron-emitting device is around a threshold of emission/non-emission of the light-emitting substance by irradiation of electrons from the electron-emitting device when the light-emitting substance is not required to emit light by irradiation of electrons from the selected electron-emitting device.
Still another aspect of the image forming method according to the present invention has the following steps.
There is provided an image forming method in an image forming apparatus having
a plurality of electron-emitting devices,
an light-emitting substance for emitting light by irradiation of electrons emitted from the electron-emitting devices,
first potential application means for sequentially selecting the plurality of electron-emitting devices and applying, to a selected electron-emitting device, a predetermined potential different from a potential applied to an unselected electron-emitting device, and
second potential application means for applying a potential corresponding to an image signal to at least a selected electron-emitting device for luminance gray scale display,
characterized in that the lowest luminance in luminance gray scale display is an light emission level.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.